This invention relates to a disk array system as one of memory devices which are used in a computer system and, more particularly, to a disk array system for monitoring access frequency to each disk drive and reallocating logical volumes in accordance with loads on the disk drives.
A disk array technique is used to improve the reliability of a memory device. Such a technique is a method proposed by David A. Patterson et al., “Report No. UCB/CSD87/391” of University of California, December, 1987. According to the method, a plurality of disk drives are grouped (hereinafter, referred to as an ECC group) and redundancy is added thereto, thereby enabling a failure to be recovered when the failure of the disk drive occurs.
According to Patterson et al., the disk arrays are classified into the following six levels in dependence on a level of reliability. At RAID level 0, data is balance-allocated to a plurality of disk drives and redundant data to recover failure data is not provided. RAID level 1 is also called a mirror ring, a perfect duplicative disk drive of one disk drive is provided, and when a failure occurs in the disk drive, processes can be executed by the duplicative disk drive. At RAID level 2, a Hamming code is used as redundant data and the redundant data and user data are interleaved to a plurality of disks.
At RAID level 3, the user data is divided on a bit or byte unit basis and the divided data is written into or read out from a plurality of disk drives in parallel. The disk drive to record the redundant data is fixedly allocated. The rotation of each disk drive is synchronized and the reading/writing operations from/into the drives are executed in parallel. At RAID level 4, the data is divided on a block unit basis and the reading/writing operations are executed to the ECC group. The disk drive to record the redundant data is fixedly allocated in a manner similar to that at RAID level 3. Unlike RAID level 3, the rotation of the disk drives is not synchronized. At RAID level 5, the data is divided on a block unit basis and the reading/writing operations are executed to the ECC group in a manner similar to that at RAID level 4. RAID level 5 differs from RAID level 4 in a respect such that the disk drive to record the redundant data is not fixedly allocated but the redundant data is recorded to all of the disk drives.
Among RAID level 0 to RAID level 5, RAID level 1 and RAID level 5 are generally used. At RAID level 5, assuming that the number of disk drives to store the data is equal to n, the data is stored into the (n+1) disk drives. Generally, when the value n becomes large, the number of disk drives increases, so that performance is improved. However, since a possibility that a failure occurs in the disk drive in the ECC group and the ECC group becomes unusable also rises, if the value n is large, the reliability of the disk array system deteriorates.